Most CVT mechanisms rely on pressure to create a frictional force between one surface and another to transfer torque from one rotating member to another.
In the case of a Toroidal CVT, traction rollers are clamped between an input and output disc; while in the case of belt or chain CVTs belt segments are clamped between similar discs. In both cases tangential force is transferred from the clamped component (roller or belt segment) to and from the discs through a special Traction Fluid.
The Traction Fluid has the unique property of increasing its viscosity when under pressure. When under high pressure of 0.5 GPa this increase is of the order of 10,000 times while when under very high pressures of 2 GPa the increase is of the order of 1,000,000,000 times. The graph in FIG. 1 illustrates this pressure dependant relationship and also its corresponding relationship with temperature.
This high viscosity allows the fluids to transmit high shearing forces between two surfaces with only small differences in speed (creep) between the two surfaces when the contact pressure between the two surfaces is high. The limitation on how large these forces can be is related to the properties of the materials used in the rolling or translating elements, and the design life of the device.
The power density of a CVT using these fluids is directly related to the relationship of the allowed tangential force to the applied clamping force. This is most often referred to as the Traction Coefficient. The maximum Traction Coefficient is the highest ratio between the tangential force and the contact force and is typically less than 0.1. When the tangential force is greater than that defined by this upper limit the contact will start to slip excessively. The heat generated by this slip reduces the viscosity and the slip increases at an exponential rate. By using a higher Traction Coefficient more torque can be transferred; but above a certain level too much creep or slip will occur and the efficiency of the CVT will suffer. Most toroidal traction drives use a traction coefficient of around 0.06-0.07.
The clamping force must be high enough to transfer the tangential forces without excessive slipping, and the tangential force must never become great enough to cause a gross breakdown of the fluid film caused by excessive slip and accompanying excessive heating.
It is not generally accepted that the clamping force remain constant and large enough to manage the highest tangential forces that could be generated in the CVT. Normally some form of Ball Ramp device is placed in the input drive that is designed to generate a clamping force that is directly proportional to the input torque. This ensures that high contact forces are only present when high torque is passing through the CVT. This significantly extends the fatigue life of the components stressed by the clamping force and the life of the Traction Fluid.
These ball ramps are designed to convert the input torque (typically from an engine) to an axial or clamping force and are for this reason placed on the input side to the CVT.
However the quantum of the tangential forces generated within the toroidal CVT mechanism is both a function of the torque and the ratio position of the rollers. Typically, with the Ball Ramp mounted on the input side, the system is adequately clamped when in low gear (when tangential forces are high) but “over clamped” when the system is in high gear.
The detailed geometry of the rollers and discs also affect the degree of over- or under-clamping. The particular geometry of the half toroidal CVT (SHTV) is suited to an input mounted ball ramp as it becomes only slightly over clamped when in high and low gear.
Elimination of over-clamping will extend the life of a CVT and improve its efficiency.
Typical Ball Ramp
A typical Ball Ramp arrangement consists of two plates that are each machined with slots that face each other and trap a ball or roller. One plate is connected to the inputted rotating energy and the other to the system being rotated. The slots consist of two ramps so that when torque is applied to the “Input Ramp Disc” the ramps force the roller against the opposing ramp machined in the “Output Ramp Disc” and create a clamping force.
The quantum of the clamping force is defined by this equation:CF=(T×1/r)/TAN θ
Where:                1. CF is the clamping force in Newtons        2. T is the input torque in Nm        3. r is the radial distance from the centreline of rotation to the centre of the roller or ball in meters        4. θ is the ramp angle in degrees        
In a typical CVT the amount of normal force required to transmit forces is given by this equation:NF=TF/μWhere                1. NF is the normal force in N        2. TF is the tangential force at the point of contact in N that must be generated between the input disc and the roller belt or chain.        3. μ is the traction coefficient        
The use of a Ball Ramp with a Toroidal CVT is relatively simple and shown in FIG. 2. FIG. 3 depicts a similar arrangement using a roller supported on an axle that bears up against a single ramp. Because the toroidal discs do not move the clamping can be executed using only mechanical interactions, provided deflections are allowed for.
The angle of the ramp is arranged to provide the right clamping force to ensure no slip occurs at the contact of the Discs and Rollers.
As noted earlier, the correct angle is derived from the formula NF=TF/μ, with TF being the maximum tangential force, which in this arrangement occurs at low gear.
As the CVT changes ratio towards a higher gear the clamping force becomes more than is needed and the discs are effectively over clamped. Only in low gear is the Traction Coefficient operating at its preferred value. This behaviour is peculiar to toroidal traction drives, other traction drives such as those that use balls or discs like a kopp variator, are most suitable for application of an input clamping ball ramp.
It is possible to design a system that uses an additional piston that can be engaged or disengaged as the ratio changes giving a stepped reduction in clamping force.
With this two stage system it is important that any reversed torque (such as that occurring during engine braking) is very low; as when the torque reverses the relative tangential forces become greater in over drive (high gear) than in under-drive. This arrangement is fundamentally unsuitable for use in a flywheel based KERS as the reversing torques are very high. It is also unsuitable when used in a multi stage IVT when the variator is swept more than once through a full ratio change.
It is an object of the present invention to create a clamping system based on simple mechanical components to create as close to ideal normal clamping forces on the roller contact points in a traction based variator.
The invention can be applied to toroidal variators, both single and half, and to other forms of variator requiring control of clamping forces.